Combined cycle power plants generally include a gas turbine, which utilizes the Brayton cycle, and a steam turbine, which utilizes the Rankine cycle. Greater efficiency may be achieved by utilizing a gas turbine and a steam turbine in combination than may be achieved by utilizing a gas turbine or a steam turbine independently. A combined cycle power plant typically includes a gas turbine, a heat recovery steam generator, and a steam turbine. A gas turbine is coupled with a generator to generate electricity. An exhaust gas from the gas turbine is introduced into the heat recovery steam generator to generate a flow of steam. The steam drives the steam turbine, which is coupled with a generator to generate additional electricity.
Minimizing start-up times improves the availability of combined cycle power plants and reduces maintenance cost and start-up emissions. The overall start-up time for a combined cycle power plant is limited by the start-up time of the steam turbine. Gas turbine start-up is fast relative to steam turbine start-up. During start up there is a relatively rapid increase in the exhaust temperature from the gas turbine. As the load of the gas turbine increases, a limit is reached on the exhaust temperature. A gas turbine controller then increases the airflow of the unit while maintaining the exhaust temperature limit. The exhaust flow and exhaust temperature is directly related to the amount of energy discharged in the heat recovery steam generator and the steam temperature generated by the heat recovery steam generator.
Steam turbine start-up is slow relative to gas turbine start-up. The start-up time of the steam turbine is limited by thermal stresses caused by temperature gradients between the rotor core and blades. These thermal stresses are monitored by measuring the temperature difference between the rotor and the steam at the inlet of the steam turbine. The allowable steam inlet temperature is limited by the rotor temperature. As the rotor temperature increases, higher inlet steam temperatures are allowed. Because the steam turbine rotor temperature sets a limit on the allowable inlet steam temperature and the gas turbine exhaust temperature controls the steam temperature, the gas turbine may not increase in load until the steam turbine rotor is heated to a sufficient temperature. This may reduce revenue by causing the power plant to operate for an extended period at a lower efficiency condition. Start-up emissions also may be increased because the load of the gas turbine may be too low for the combustor to operate in an efficient manner, thus causing concentrations of emissions such as NOx and CO to be greater than they would at higher load conditions.
A method and apparatus for warming a steam turbine during start-up of the gas turbine is desirable in order to reduce thermal stresses and decrease start-up times of a combined cycle power plant.